The present invention relates generally to a thin electroluminescent (EL) display element. More particularly, it pertains to a thin film EL display element exhibiting short wavelength visible light or near ultraviolet emission characteristics and providing improved luminescent efficiency, high luminance, and stability.
In the area of information displays, cathode ray tube phosphors have proven themselves to be outstandingly efficient light emitters with excellent color capability. These types of displays, however, like the common picture tube, require an inordinate amount of space, especially when mated with ever-shrinking electronics. Flat panel display systems, such as those using a liquid crystal display panel with photoluminescent phosphor lamps, have grown in popularity, but lack brightness. A viable alternative to liquid crystal displays is the use of electroluminescent displays which offer increased brightness.
Thin film alternating current (AC) electroluminescent (EL) displays comprise a matrix of solid-state, wide-angle light emitting pixels or display elements. Amber light emitting ZnS:Mn EL phosphors have been the mainstay in EL displays for years, limiting the EL display range of color and, therefore, their acceptance. Rare-earth doped alkaline earth sulfide EL phosphors have been employed more recently in an effort to produce light over a wider range of colors within the visible spectrum, including realization of full-color displays, but with limited success.
An alternating current (AC) thin film electroluminescent TFEL display structure capable of emitting light of different colors typically has a capacitor-like structure as shown schematically in FIG. 1 and further detailed in A. H. Kitai, Solid State Luminescence: Theory, Materials and Devices. Chapman and Hall (1993) pp. 229-262. The display is made up of a transparent glass substrate 10, a transparent bottom electrode set 12 comprised of elongate conducting members, a first insulator layer 14, a luminescent semiconductor layer 16, and a second insulator layer 18, each layer formed on top of the other on the glass substrate. A top electrode set, such as conducting strips 20a and 20b, is deposited on top of the second insulator layer. Upon application of a strong electric field across the top and bottom electrode sets, the luminescent layer 16 at the intersection of each pair of energized electrodes (defining a display element) emits light which emanates through the transparent glass substrate 10.
Matrix-addressable TFELs are conventionally constructed by positioning the top and bottom electrode sets orthogonal to one another as described in U.S. Pat. No. 4,719,385 to Barrow et al and shown in FIG. 1. One way in which a plurality of colors may be displayed by a thin film EL device is by stacking such devices on top of one another, each containing a phosphor which emits a different color, thereby allowing different color combinations. A second arrangement is shown in U.S. Pat. No. 4,894,116 also to Barrow et al. which places the different phosphor materials emitting light in the three primary colors of red, green and blue in adjacent parallel strips within the electroluminescent layer, each color being addressable by electrodes placed in a matrix array. Whatever the arrangement, a need remains for a blue phosphor having both color purity and increased luminosity for the adequate formation of color combination schemes of the three primary colors.
In the foregoing thin film EL element having two dielectric layers, the luminescent layer 16 is made of a suitable luminescent host material doped with one or more impurities to form a luminescent center. A strong electric field applied to the luminescent layer 16 excites the electron energy level of the luminescent center. When the excited state returns to the ground state, the conversion of energy into light takes place. The result is electroluminescence.
More particularly, electroluminescence is thought to result from two mechanisms: electron impact and recombination. Electron impact occurs when a free electron, moving under a force generated by an external electric field, collides with another electron which resides in the dopant material. The impacting electron transfers energy to the other electron which consequently raises the impacted electron to a higher energy level. Because this new energy level is unstable, the electron soon de-excites thereby releasing the absorbed impact energy as luminescent light. Since luminescence by electron impact occurs when free electrons are accelerating under a force, electron impact luminescence is generally detected only in the presence of an active electric field. Thus, we would expect to see this type of mechanism during the leading edge of voltage pulses applied across the EL device as in FIG. 6(b).
Recombination results when free electrons moving through the EL material under an electric field are captured by the dopant atoms when the field is turned off and the electrons slow down. Recombining electrons consequently fall into a certain energy level and release their excess energy as photonic light. It is expected that this type of emission occurs at the trailing edge of voltage pulses as shown in FIG. 6(c).
For electroluminescence to take place, it is necessary that a strong electric field be applied to the luminescent layer and that the luminescent host material consequently have a broad band gap. A luminescent host material having a narrow band gap does not withstand the strong electric field applied to the luminescent layer. For this reason, conventional luminescent host materials have been selected from II-VI compounds, such as ZnS, SrS, CaS, and ZnSe, which have a broad band gap. The conventional luminescent center doping is typically either manganese (Mn) or europium (Eu), but can include such materials as samarium (Sm), terbium (Tb), cerium (Ce), and praseodymium (Pr).
Luminescent materials are also selected for their particular color emissions. The light emitted by electroluminescence has a specific wavelength which depends on the material used for the luminescent center. By combining the proper luminescent materials within a single device, the resulting color emission combinations could be suitable for multi-color display systems.
In order to form a white phosphor or otherwise obtain a full color spectrum display, the three primary colors red, green, and blue are combined. Heretofore, however, only electroluminescent materials which emit in the lower energy half of the visible light spectrum have been bright enough or stable enough for use in fiat panel displays. Typical examples of electroluminescent materials used for displays include europium-doped calcium sulfide (CaS:Eu) which emits red light, europium-doped strontium sulfide (SrS:Eu) which emits orange-red light, manganese-doped zinc sulfide (ZnS:Mn) which emits yellowish orange light (although this phosphor material is generally filtered to obtain a bright red color), and terbium-doped zinc sulfide (ZnS:Tb) which emits green light. Unfortunately, blue and violet light emitting phosphors have generally weak light outputs, are expensive to produce, and are not durable for long term use.
It is advantageous for full color displays to yield light which is visible across as much of the spectrum to which the human eye is sensitive as possible. The human eye can only see light within the so-called visible spectrum, which consists of light with wavelengths from 400 to 700 nm. The CIE chromaticity curve 21, shown in FIG. 2, plots in two dimensions the light sensitivity range of the human eye wherein pure, essentially monochromatic light, lies on the curve 21 of the diagram. Impure or mixed hues visible to the human eye reside within curve 21. For example, ZnS:Mn emits yellowish-orange light with coordinates x=0.53, y=0.47 as shown at 22 and ZnS:Tb emits green light with coordinates x=0.31, y=0.60 as shown at 24. These values are approximate and can be altered depending upon the dopant concentration used in the phosphor material. In a di-phosphor compound using those two EL materials, one may obtain all colors which fall on line 26 connecting the two points 22, 24. By adding a third element, for example SrS:Ce with a blue-green chromaticity (x=0.20, y=0.36) shown at 27, one can obtain all color combinations which fall within the area shown at 28 and bounded by dashed-dot lines connecting the points 22, 24, 27. The greater the area, the more natural colors which can be obtained and displayed on the color display. It is thus advantageous to have an EL material which emits as close to the CIE boundary as possible to yield the maximum range of color combinations.
Fluorescent die organic compounds have been proposed for use within the emitting layer of an organic EL device to absorb EL emitted blue light and consequently emit a fluorescence in the bluish-green to red spectrum. Such a compound is disclosed in U.S. Pat. No. 5,126,214 to Tokailin et al. The organic material suggested by Tokailin et al., however, entails a complicated structure and provides concerns not existent in inorganic EL materials.
Alkaline earth sulfides have been proposed as blue emitters in a thin film electroluminescent (TFEL) panel as shown in Barrow et al. U.S. Pat. No. 4,751,427. The Barrow et al. patent discloses the use of strontium sulfide (Srs) as a host material doped with cerium fluoride (CeF.sub.3) acting as an emitter to provide a source of photons. The problem with SrS:Ce, however, is that it has a blue-green chromaticity, which means that it is necessary to use a blue filter in conjunction with this material to achieve a blue chromaticity. When a filter is used, the luminance level is reduced to less than 15% of the original luminance. In addition, the luminance of this material tends to diminish dramatically as a function of time. The SrS material is hygroscopic and chemically unstable which adds complexity to the panel fabrication. Other blue-emitting phosphors have been investigated such as SrGa.sub.2 S.sub.4 :Ce, disclosed in Sun et al. U.S. Pat. No. 5,309,070, which has a blue color with coordinates x=0.15, y=0.10. Other conventional blue-emitting phosphors are shown as unnumbered points on FIG. 2 and include CaGa.sub.2 S.sub.4 :Ce, SrS:Ce, K and SrS:Ce. However, use of previous blue phosphors have only limited capability to emit shorter-wavelength violet light.
Accordingly, a need remains for luminosity stable blue-violet phosphor for use in electroluminescent fiat panel displays.